1. Technical Field
The present invention relates to a device and method for dissipating electrical energy during a surgical procedure and, more particularly, to an energy dissipation device and method having application in endoscopic surgical procedures.
2. Description of the Related Art
Endoscopic procedures have been practiced for many years. Initially, endoscopic procedures were primarily diagnostic in nature, i.e., involving the introduction of laparoscopes and like devices to view internal organs. Laparoscopic sterilizations have also been performed for some time. More recently, surgeons have begun to perform complex and innovative endoscopic surgical procedures. For example, endoscopic cholecystectomy has become a preferred modality for gall bladder removal in just the last two years.
Endoscopic procedures are typically performed through trocar sleeves or cannulas. Prior to introducing the cannula through the body wall, the surgeon generally insufflates the body cavity with carbon dioxide, e.g., through a Verres needle or like device. Insufflation creates a free area between internal body organs and the body wall. The surgeon then introduces one or more trocars through the body wall into the insufflated body cavity.
Trocars are sharp pointed instruments which include a trocar shaft or obturator with a piercing tip located at the distal end thereof. The trocar is positioned within a cannula for introduction of both into the body cavity. After introduction, the trocar is removed from the cannula and endoscopic instrumentation may be introduced through the cannula to effect diagnostic and/or surgical procedures. The cannula is typically provided with an internal flapper or trumpet valve to prevent gas release from the body cavity and generally measures from 3 to 12 mm in diameter. Safety trocars which include a spring-loaded safety shield surrounding the obturator are also known. See U.S. Pat. No. 4,601,710 to Moll et at.
A wide range of accessory instrumentation finds application in endoscopic surgical procedures. For example, graspers, dissectors, clip appliers, lasers and electrocautery devices are routinely employed. It is also common to perform a cholangiogram by introducing a catheter into the cystic duct during an endoscopic cholecystectomy. To obtain high quality imaging, radiolucent (rather than stainless steel) cannulas are typically employed in the region of the cholangiogram. Tissue gripping devices have also been developed for deployment on the exterior of the cannula, either permanently or removably, to prevent the cannula from working out of the body wall as a result of instrument manipulation therethrough.
Surgeons have long employed cautery to destroy or vaporize tissue and to cut and coagulate vessels and tissue through the application of electricity, heat or corrosive chemicals. Electrocautery (or galvanocautery) is routinely used to achieve hemostasis, both in endoscopic (closed) and non-endoscopic (open) surgical procedures. Electrocautery operates by taking advantage of the resistance to the flow of electrical current through tissue, such resistance resulting in heat generation as current attempts to pass therethrough. The degree of resistance to electrical current flow for tissue depends primarily on its vascularity and water content, with bone and fat having a higher resistance to current flow than skin and muscle.
Electrocautery instrumentation is typically either unipolar or bipolar. In unipolar applications, electrical energy is supplied from a generator to the end of an electrical conductor, e.g., cauterization wire, which is pressed against or placed adjacent the desired surgical site. A grounding plate is typically located below the patient so that the electrical current delivered by the cauterization wire passes through the patient to the plate, thereby completing the electrocautery circuit. In bipolar electrocautery, by contrast, a grounding wire is pan of the instrumentation placed adjacent the surgical site and the electrical current passes directly from the cauterization wire, through the tissue at the surgical site, and to the grounding wire.
Electrical burns constitute the most common electrical hazard in the operating room, the majority of which are associated with the use of electrosurgery or cautery units. A radio frequency generator supplies alternating current to the active electrode tip at frequencies between 300 thousand and 2 million hertz (cycles/second). Higher frequency devices are preferred to minimize the likelihood of muscle and nerve stimulation, i.e. electrical shock. The total power output of such units typically ranges from 40 to 600 watts and the units supply up to 10,000 volts. The current density at the tip of the electrode is extremely high, allowing the surgeon to achieve local heating and coagulation. To minimize the risk of extraneous burns in endoscopic procedures, it has been suggested that low power electrosurgical units and bipolar gasping forceps be employed and that insulation of accessory instrumentation, the cannula and the laparoscope be undertaken. See "Principles and Hazards of Electrosurgery Including Laparoscopy," G. R. Neufield, Surgery, Gynecology & Obstetrics, November 1978.
It has been reported that conductive trocar sleeves (or cannulas) can result in internal bums, particularly where the operative portion of the electrocautery instrument is not advanced beyond the end of the cannula. The suggested solution to this risk is the use of non-conductive cannulas, e.g., by placing a polyolefin heat-shrinkable tubing on the exterior of a conductive, stainless steel cannula. See "A Method for Preventing Abdominal Bums Caused by Electrocautery during Laparoscopy," J. M. Esposito, Am. J. Obstetr. Gynecol., Vol. 114, No. 8, 1972, pages 1105-06.
It has also been reported that endoscopic use of electrocautery devices can induce an electrical charge in or on the endoscope through which the electrocautery device is operated, despite the insulation of the electrocautery device. It will thus appear that high frequency current is leaking through the insulation, when in fact no electrons are passing therethrough. This phenomenon has been described as "capacitive coupling." See "Endoscopic Applications of Electrosurgery: A Review of Basic Principles," D. E. Barlow, Gastrointestinal Endoscopy, Vol. 28, No. 2, 1982, pages 73-76; "Safeguards in Laparoscopy: Education, Equipment Care, and Electron Control," R. M. Soderstrom, Contemporary OB/GYN, Vol. 11, March 1978, pages 95-107.
The induction of electric charge has also been observed on the surface of nonconductive cannulas when used endoscopically with a unipolar cautery. The electrons passing through the unipolar cautery create an electromagnetic field which in turn creates an electric charge in the nearby cannula. The electric charge thus created is proportional to the frequency of the energy supplied to the cautery device. This electrical charge may be stored and released through a small area, such as the cannula touching the bowel, resulting in a bum. An electromagnetic field is not generally created when a bipolar cautery is used because electric currents run in both directions within the cannula, so that the net electromagnetic field is zero. In the case of unipolar cautery, unless this electrical energy is dissipated, contact between the "charged" cannula and an internal body organ or viscera may bum the organ or viscera.
Thus, means are needed to ensure that any electrical energy induced in or in any way conveyed on to surrounding structures by the use of electrosurgical devices is controllably dissipated in a safe manner.